This invention relates to motor vehicle lighting adjustment systems.
Every motor vehicle has a headlamp system adapted to provide at least two different forms of lighting -- a long-range or "main" beam and a medium-range or "dipped" beam.
The main beam is designed to give maximum-intensity lighting of the road a considerable distance ahead of the vehicle, whereas the dipped beam is the result of a compromise between the desire to see as far as possible and the need not to dazzle the drivers of oncoming vehicles. Conventionally, the dipped-beam adjustment is such that beam cut-off has a 0.01 radian dip, corresponding to 35 minutes of an angle, below the horizontal -- i.e., a dip of 25 cm in 25 m. On the assumption that the headlamps producing the dipped beam are 75 cm above ground level, the theoretical range of the beam would be 75 meters on the left of the vehicle and the order of 100 meters on the right (there being a lateral as well as a downward tilt of the beam). In practice, the actual visibility range provided by halogen bulb headlamps is from 70 to 80 meters on the right and from 40 to 50 meters on the left.
However, when a driver changes from main beam to dipped beam illumination, for instance, when passing another oncoming vehicle, the abrupt and considerable reduction in illumination of the road from 50 to 150 meters ahead of the vehicle causes the driver to see a black patch which at least for a few seconds greatly reduces the range of visibility normally afforded on dipped-beam illumination; the changeover from main beam to dipped beam illumination causes a four-fold reduction in illumination of the road 70 meters ahead of the vehicle and, because of persistence of vision effect in the driver, causes a doubtlessly brief but nevertheless very considerable reduction in the maximum range of visibility.
A reduction in visibility of this nature is of course likely to cause accidents since the driver has no chance of avoiding a hazard seen too late. It also greatly disturbs driving convenience.